Abtract <p>One of the practical problems in the oil and gas industry is the formation of oil–acid emulsions and asphaltene sludge when well stimulation fluids come in contact with oil. The key to solving these problems lies in understanding the processes occurring at the interfaces between oil and these agents, with these processes being qualitatively studied in this work. This research investigates the processes occurring between sensitive oil and two types of well stimulation fluids based on hydrochloric acid and ethylenediaminetetraacetic acid (EDTA) in the presence and absence of a surfactant. Three following methods have been employed: simple mixing (“bottle-test”), simultaneous flow of fluids in a capillary, and simultaneous flow of fluids in a micromodel simulating a porous medium. Using the simple mixing method, it has been shown that the addition of the surfactant to a 15% hydrochloric acid solution can prevent sludge formation but does not hinder emulsion formation. Additionally, it has been found that EDTA-based fluids with neutral pH values are compatible even with sensitive oil. Experiments on the simultaneous flow have shown the role of capillary walls in creating sludge and the cleaning ability of the chelating agent. The simultaneous flow in the micromodel has revealed the distinctive features of hydrochloric acid as compared with the chelate, i.e., the discrete flow, emulsion formation in the near-wall layer, and precipitate formation on pore walls. The results of this study may be useful both for further fundamental studying interfacial processes in oil reservoirs and for practical applications.</p>